Devices employing liquid crystals have found use in a variety of electrooptical applications, in particular those which require compact, energy-efficient, voltage-controlled light valves, e.g., watch and calculator displays, as well as the flat-panel displays found in portable computers and compact televisions. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them the most promising of the non-emissive electrooptical display candidates currently available.
One of the most important characteristics of a liquid crystal display device is its response time, i.e., the time required for the device to switch from the on (light) state to the off (dark) state. In a ferroelectric or anti-ferroelectric device, response time (.tau.=.eta.sin.sup.2 .THETA./P.sub.s E) is proportional to the rotational viscosity (.eta.) of the liquid crystal compound(s) contained within the device, is also proportional to the square of the sine of the cone tilt angle (.THETA.)) of a tilted smectic mesophase of the compounds, and is inversely proportional to the polarization (P.sub.s) of the compounds and to the applied electric field (E). Thus, response time can be reduced by using compound(s) having high polarizations and/or low viscosities and/or low cone tilt angles, and such compounds are greatly desired in the art.
Other important characteristics of a liquid crystal display device are its brightness and contrast ratio. High brightness and contrast ratios provide enhanced optical discrimination and viewing ease and are therefore preferred. Brightness is related to the intensity of light transmitted through a device, which for a surface-stabilized ferroelectric device (as described in U.S. Pat. No. 4,367,924, the description of which is incorporated by reference herein) with two polarizers can be represented by the equation EQU I=I.sub.o (sin.sup.2 (4.THETA.)) (sin.sup.2 (.pi..DELTA.nd/.lambda.)),
where I.sub.o =transmission through parallel polarizers, .THETA.=liquid crystal cone tilt angle, .DELTA.n=liquid crystal birefringence, d=device spacing, and .lambda.=wavelength of light used. The maximum transmission is obtained when both the terms sin.sup.2 (4.THETA.) and sin.sup.2 (.pi..DELTA.nd/.lambda.) are at a maximum (each term equals one). Since the first term is at a maximum when the liquid crystal composition in the device has a cone tilt angle of 22.5 degrees, liquid crystal compounds which have cone tilt angles close to 22.5 degrees (or which can be mixed with other liquid crystal compounds to form compositions having cone tilt angles close to 22.5 degrees) are also highly desired in the art.
In particular, since many fluorine-containing liquid crystal compounds have cone tilt angles which exceed the optimum value of 22.5 degrees, materials and methods for reducing cone tilt angle are needed. Although hydrocarbon liquid crystal compounds have low cone tilt angles (below 22.5 degrees), they generally cannot be used for this purpose due to their incompatibility with fluorine-containing liquid crystal compounds (which generally leads to loss of the active mesophase).
In addition to fast response times and optimized tilt angles, liquid crystal compounds should ideally possess broad smectic temperature ranges (to enable operation of a display device over a broad range of temperatures) or should be capable of combination with other liquid crystal compounds without adversely affecting the smectic phase behavior of the base mixture.